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Power supply poor performance.

P

pawihte

Jan 1, 1970
0
I tried my hand at making a 9V power supply with an MC34063A. I
get the correct DC voltage output but was disappointed with the
very dirty output. I know that, in general, simple switched-mode
PSes have poorer performance than linear types, but what I
observed was worse than I expected from the sample circuit given
in the datasheet. This is the schematic, along with the
single-sided pcb layout (in case it's due to poor layout):
http://img683.imageshack.us/img683/4302/9vsmps.png

My main scope is out of order and I used my backup 15MHz
single-trace analog scope. It shows narrow spikes of unsteady
amplitude, varying from roughly +1V/-0.5V to +2/-1V around the dc
level. Moreover, the frequency of about 15 kHz is much lower than
I expected.

The spike amplitudes were first observed without the second L-C
filter. Adding that made little difference at the output of the
first filter, and only a slight reduction at the output of the
second filter. The load was the LED plus a 470-ohm resistor
(total 24mA).

I used general-purpose caps (ESR unknown) for the output filters.
Paralleling them with non-electrolytic plastic and ceramic caps
of 0.1uF have no discernible effect. The timing cap is a ceramic
disc that shows 465pF on my LCR meter. I wound the inductors with
23 swg (~22 awg) enamelled Cu wire on ferrite ring cores.

What am I doing wrong? Is it the filter caps, poor PCB layout or
something else?
 
A

Archimedes' Lever

Jan 1, 1970
0
I tried my hand at making a 9V power supply with an MC34063A. I
get the correct DC voltage output but was disappointed with the
very dirty output. I know that, in general, simple switched-mode
PSes have poorer performance than linear types, but what I
observed was worse than I expected from the sample circuit given
in the datasheet. This is the schematic, along with the
single-sided pcb layout (in case it's due to poor layout):
http://img683.imageshack.us/img683/4302/9vsmps.png

My main scope is out of order and I used my backup 15MHz
single-trace analog scope. It shows narrow spikes of unsteady
amplitude, varying from roughly +1V/-0.5V to +2/-1V around the dc
level. Moreover, the frequency of about 15 kHz is much lower than
I expected.

The spike amplitudes were first observed without the second L-C
filter. Adding that made little difference at the output of the
first filter, and only a slight reduction at the output of the
second filter. The load was the LED plus a 470-ohm resistor
(total 24mA).

I used general-purpose caps (ESR unknown) for the output filters.
Paralleling them with non-electrolytic plastic and ceramic caps
of 0.1uF have no discernible effect. The timing cap is a ceramic
disc that shows 465pF on my LCR meter. I wound the inductors with
23 swg (~22 awg) enamelled Cu wire on ferrite ring cores.

What am I doing wrong? Is it the filter caps, poor PCB layout or
something else?
Yes.
 
T

Tim Williams

Jan 1, 1970
0
pawihte said:
My main scope is out of order and I used my backup 15MHz single-trace
analog scope. It shows narrow spikes of unsteady amplitude, varying from
roughly +1V/-0.5V to +2/-1V around the dc level. Moreover, the frequency
of about 15 kHz is much lower than I expected.

Scope shot?

More importantly, what's pin 2 look like? And the current through R1?
I used general-purpose caps (ESR unknown) for the output filters.
Paralleling them with non-electrolytic plastic and ceramic caps of 0.1uF
have no discernible effect. The timing cap is a ceramic disc that shows
465pF on my LCR meter. I wound the inductors with 23 swg (~22 awg)
enamelled Cu wire on ferrite ring cores.

Aha! Ferrite is shit for DC filtering. You're probably saturating them.
Try a ferrite rod or powdered iron toroid.

How many turns? What's "300uH" based on, is it measured? At what bias?
What am I doing wrong? Is it the filter caps, poor PCB layout or something
else?

Layout looks pretty good. D5 and C2 seem to be on the wrong sides, C2
should be closer to the chip I guess, but with L1 where it is, that might be
tricky. Maybe it can get just close enough to that mounting hole, or see
what it does rotated. Anyway, that ground plane everything's connected to
is pretty wide and this should make very little difference. I'd say it's a
good tight layout.

Tim
 
M

Martin Riddle

Jan 1, 1970
0
pawihte said:
I tried my hand at making a 9V power supply with an MC34063A. I get
the correct DC voltage output but was disappointed with the very dirty
output. I know that, in general, simple switched-mode PSes have poorer
performance than linear types, but what I observed was worse than I
expected from the sample circuit given in the datasheet. This is the
schematic, along with the single-sided pcb layout (in case it's due to
poor layout):
http://img683.imageshack.us/img683/4302/9vsmps.png

My main scope is out of order and I used my backup 15MHz single-trace
analog scope. It shows narrow spikes of unsteady amplitude, varying
from roughly +1V/-0.5V to +2/-1V around the dc level. Moreover, the
frequency of about 15 kHz is much lower than I expected.

The spike amplitudes were first observed without the second L-C
filter. Adding that made little difference at the output of the first
filter, and only a slight reduction at the output of the second
filter. The load was the LED plus a 470-ohm resistor (total 24mA).

I used general-purpose caps (ESR unknown) for the output filters.
Paralleling them with non-electrolytic plastic and ceramic caps of
0.1uF have no discernible effect. The timing cap is a ceramic disc
that shows 465pF on my LCR meter. I wound the inductors with 23 swg
(~22 awg) enamelled Cu wire on ferrite ring cores.

What am I doing wrong? Is it the filter caps, poor PCB layout or
something else?

You might want to experiment with Tantalums for C1, C3.
And looking at your layout there is potential of coupling the input
switching noise to the feed back pin. A small capacitance might help,
with out affecting loop response.
Also this type of switcher creates a lot of hash, so a input filter of
some type (LC) is usefull.

Cheers
 
P

pawihte

Jan 1, 1970
0
Martin said:
You might want to experiment with Tantalums for C1, C3.
And looking at your layout there is potential of coupling the
input
switching noise to the feed back pin. A small capacitance might
help,
with out affecting loop response.
Also this type of switcher creates a lot of hash, so a input
filter of
some type (LC) is usefull.

Thanks for the reply. I'll try out your suggestions and see what
happens.
 
P

pawihte

Jan 1, 1970
0
Tim said:
Scope shot?

More importantly, what's pin 2 look like? And the current
through R1?

OK. I'll upload the shots but it'll have to wait a bit.
Aha! Ferrite is shit for DC filtering. You're probably
saturating
them. Try a ferrite rod or powdered iron toroid.
Ah. I'm not completely ignorant about the saturation thing, but I
have limited experience with unbalanced filtering and it just
didn't cross my mind. Thanks for the heads-up.
How many turns? What's "300uH" based on, is it measured? At
what
bias?
12 turns. Measured without dc bias. The core is something I
salvaged from junk.
Layout looks pretty good. D5 and C2 seem to be on the wrong
sides, C2
should be closer to the chip I guess, but with L1 where it is,
that
might be tricky. Maybe it can get just close enough to that
mounting
hole, or see what it does rotated. Anyway, that ground plane
everything's connected to is pretty wide and this should make
very
little difference. I'd say it's a good tight layout.
I could move C2 to the copper side and solder it directly to the
IC pin but, as you said, I wouldn't expect that to make a lot of
difference. Thanks for the reply.
 
T

Tim Williams

Jan 1, 1970
0
mook johnson said:
7) (this is a biggie) When probing for noise connect the tip of the probe
to the ground clip and touch this to board ground.
If you're seeing a lot of noise there is a lot of common mode noise
between the board ground and earth ground.

The ol 'shorted' probe test?

This is more about RFI in general than common mode specifically. When it
switches, current loops and voltage loops throw off electromagnetic
radiation. With the probe anywhere in the near field, you'll pick up a
delicious burst of switching noise. The solution is a coax probe, where
possible. For instance,
http://webpages.charter.net/dawill/Images/RegBO1.jpg
The red/black twisted pair running up left of center goes to a 0.47 ohm
current sense resistor. It sees this waveform:
http://webpages.charter.net/dawill/Images/RegBO2.jpg
regardless of how the 10x probe is connected (notice the clip with wire
above the heatsink, and its ground clip coming across above the
transformer).

If I tried watching the same current with the 10x probe, it would be
unrecognizable from all the trash. Notice the complete absence of ringing
and hash when the transistor switches, or when the secondary diode turns off
(discontinuous mode).

Tim
 
J

Jon Kirwan

Jan 1, 1970
0
Thanks for the reply. I'll try out your suggestions and see what
happens.

I'm especially interested in the inductor core situation. If
you do change it, and it helps, let us know. There are some
other good points made but I thought I'd focus on the core,
because I'm still learning about these details and wanted to
consider it a little.

You mentioned 300uH measured and 24mA load and 12 turns on
the inductor. With ferrite, let's use a Bsat = 0.1T to be
safer. We assume not to know mu_r, for now. That still
tells a lot.

L = mu_0 * mu_r * N^2 * A_e / l_m

but also,

Bsat = mu_0 * mu_r * N * I / l_m

This second equation can be solved to place the unknowns on
one side and the knowns on the other:

(mu_r / l_m) = Bsat / (mu_0 * N * I)

That can be stuffed into L, as:

L = N * A_e * Bsat / I

But we know L, so solve for the unknown A_e as:

A_e = L * I / (N * Bsat)

Stuffing in L=300e-6, I=24e-3, N=12, and Bsat=0.1 yields a
value for A_e of 6e-6 m^2. Assuming a circular profile, this
is about a radius of 1.4mm or a diameter of 2.8mm. However,
I don't imagine that the average load current is the peak
inductor current. So this is actually too small to avoid
saturation -- the diameter will need to be greater, I think.

L goes proportional to N^2 while B goes proportional to N, so
if you reduce the mu_r of your core you will only have to
wind more turns by the ratio of change in mu_r and reduces B
allowing more Ipeak. So you might consider finding a
significantly lower mu_r core and winding a few more turns to
get back to your L value. Here, I'm talking about L1, I
think. L2 looks like part of a filter to me so I have little
comment about that, now.

Jon
 
J

Jon Slaughter

Jan 1, 1970
0
pawihte said:
I tried my hand at making a 9V power supply with an MC34063A. I
get the correct DC voltage output but was disappointed with the
very dirty output. I know that, in general, simple switched-mode
PSes have poorer performance than linear types, but what I
observed was worse than I expected from the sample circuit given
in the datasheet. This is the schematic, along with the
single-sided pcb layout (in case it's due to poor layout):
http://img683.imageshack.us/img683/4302/9vsmps.png

My main scope is out of order and I used my backup 15MHz
single-trace analog scope. It shows narrow spikes of unsteady
amplitude, varying from roughly +1V/-0.5V to +2/-1V around the dc
level. Moreover, the frequency of about 15 kHz is much lower than
I expected.

The spike amplitudes were first observed without the second L-C
filter. Adding that made little difference at the output of the
first filter, and only a slight reduction at the output of the
second filter. The load was the LED plus a 470-ohm resistor
(total 24mA).

I used general-purpose caps (ESR unknown) for the output filters.
Paralleling them with non-electrolytic plastic and ceramic caps
of 0.1uF have no discernible effect. The timing cap is a ceramic
disc that shows 465pF on my LCR meter. I wound the inductors with
23 swg (~22 awg) enamelled Cu wire on ferrite ring cores.

What am I doing wrong? Is it the filter caps, poor PCB layout or
something else?

You did attach a load to the output? Compare the input to the regulator to
the output at no load and then try adding a load of maybe 10% of max and see
if it does any better. I don't see why you need a second L-C stage. The led
may not be a large enough load.

Also, your circuit seems a bit different than the buck in the datasheet.
Some of your resistor values are different(not sure if thats intentional or
not). Other than the the circuit looks correct.

On page 7 they give the characteristics of that circuit so you should be
seeing approximately the same(again, some of your component values are
different(R2 and R3).

Try a larger load of at least 100mA or even shorted and see what you get. As
has been mentioned, in an smps design, the output depends on the duty cycle.
For small loads it is non-linear and the duty is significant. For large
loads the regulation is almost independent of the duty. Hence the first
thing is try a larger load and see if that improves anything.
 
H

Hammy

Jan 1, 1970
0
I tried my hand at making a 9V power supply with an MC34063A. I
get the correct DC voltage output but was disappointed with the
very dirty output. I know that, in general, simple switched-mode
PSes have poorer performance than linear types, but what I
observed was worse than I expected from the sample circuit given
in the datasheet. This is the schematic, along with the
single-sided pcb layout (in case it's due to poor layout):
http://img683.imageshack.us/img683/4302/9vsmps.png

My main scope is out of order and I used my backup 15MHz
single-trace analog scope. It shows narrow spikes of unsteady
amplitude, varying from roughly +1V/-0.5V to +2/-1V around the dc
level. Moreover, the frequency of about 15 kHz is much lower than
I expected.

The spike amplitudes were first observed without the second L-C
filter. Adding that made little difference at the output of the
first filter, and only a slight reduction at the output of the
second filter. The load was the LED plus a 470-ohm resistor
(total 24mA).

I used general-purpose caps (ESR unknown) for the output filters.
Paralleling them with non-electrolytic plastic and ceramic caps
of 0.1uF have no discernible effect. The timing cap is a ceramic
disc that shows 465pF on my LCR meter. I wound the inductors with
23 swg (~22 awg) enamelled Cu wire on ferrite ring cores.

What am I doing wrong? Is it the filter caps, poor PCB layout or
something else?
I have a small 0.5W NCP3063 pcb inverting. Heres a shot of the input
and output ripple below.

Is this what your output looks like?

http://i50.tinypic.com/2l9l0zl.png

That's just using a bead and a cheap 100uf AL cap for my post LC
filter. Yellow is input ripple; blue is output ripple 38mVpp full
load. Are you talking about those skinny spikes riding the ripple on
the blue trace? I'm measuring this with a short ground pin on my probe
and right over a 0.1uf X7R 0603 cap right at the output.

You're always going to have weird ripple on Hysteric controllers but
yours does sound really high. The inductance on your post LC filter
seems unnecessarily large as well.

The advice mook gave is good particularly when measuring with your
scope.Your probe acts like an antenna and picks up switching noise.

Try holding your probe in the air and keep moving it closer and you
will see waveforms on your scope without even touching the circuit.
 
T

Tim Williams

Jan 1, 1970
0
Jon Kirwan said:
You mentioned 300uH measured and 24mA load and 12 turns on
the inductor.

That implies A_L = 2.08 uH/T^2 (unbiased), which is pretty high, typical for
an ungapped high-mu ferrite toroid. They're typically around 1-10 amp-turns
(AT) saturation, pretty easy to saturate. If the peaks are four times
higher than average current, that's easily 0.024A * 12T * 4 = 1.15AT, which
might be enough to saturate it. If it's highly discontinuous, the peaks
could be much higher, bringing it into saturation. And if it is
discontinuous, that could explain the unusually low frequency.
With ferrite, let's use a Bsat = 0.1T to be
safer. We assume not to know mu_r, for now. That still
tells a lot.

<snip>

Bsat ~ 0.4T is more typical, though you might want to drop it to 0.2 or 0.1
for better linearity, or for high frequency use (transformer duty only, DC
choke is different). I usually go with ~0.2T, which is a practical
factor-of-2 headroom, just so I can say I've made the allowance.

Notice that assuming peak B is equivalent to a certain amount of applied
voltseconds. This is simply because EMF = -dB/dt * A_e, or Vs = -B * A_e.
In terms of easy-to-measure parameters, you can get AT(sat) and A_L from the
inductor's V-I curve, which gets you V*s = AT(sat) * A_L / N.

I like to work with Vs because it's more useful to circuit analysis. How
many turns do I need? Integrate voltage over a quarter wave period to get
V.s (it's usually a square wave, so that's just Vpk * 4/f), and I've already
measured the core's A_L and AT(sat), so just divide and you get turns.

In this case, if AT(sat) = 1AT and A_L = 2uH/T^2, then Vs = 2*1 * 12 =
24uVs. At 9V, that's only 2.7us on-time. Pawihte, are you seeing ~3us wide
pulses?
...
That can be stuffed into L, as:

L = N * A_e * Bsat / I

Notice that mu_r disappears -- that means it doesn't matter how much
magnetization you apply, it's the volts and time that gets it up to
saturation. (You could do the same to a powdered iron core (let's say
0.1uH/T^2 and 300AT(sat)), although magnetization current gets absurdly
large for transformer duty!)
So you might consider finding a
significantly lower mu_r core and winding a few more turns to
get back to your L value. Here, I'm talking about L1, I
think. L2 looks like part of a filter to me so I have little
comment about that, now.

L2 has to carry the same current, so it'll need the same characteristics,
although it has less delta B, which basically means it can be lossier =
cheaper (one of those ugly yellow/white toroids?).

Powdered iron cores that size are in the 80nH/T^2 range, so you need about
sqrt(300 / 0.08) = 61 turns to get there. Saturation is in the >200AT
range, so you can safely dump over 3A though it -- more likely you won't
even be able to get enough turns inside it to see it saturate before I^2*R
losses overwhelm it.

Tim
 
T

Tim Williams

Jan 1, 1970
0
Hammy said:
The advice mook gave is good particularly when measuring with your
scope.Your probe acts like an antenna and picks up switching noise.

Oh, I remembered something else about probes and grounding:

If you put the probe itself through a ferrite bead, you can observe its
effect, if any. Ideally, this won't change anything. If it changes, then
you have current going somewhere it shouldn't, either up through the mains,
or between probes (if you're using more than one).

Last time I did this and saw the effect, I got:
- No bead: fast ringing, medium amplitude, medium decay
- With bead: slow ringing, same amplitude, medium decay
- Damped bead: slow ringing, low amplitude, fast decay

Bead setup: three turns through large ferrite bead. Last test: 10 ohm
resistor soldered in place through the ferrite bead (so it's acting like a
leaky shorted turn).

If you are getting probe current, adding a ferrite bead doesn't really help
your measurement, but it does change it, so you can at least guess what
hides behind the "probe cable resonance" jigglies.

BTW, I happen to have some 3" high-mu ferrite toroids, which are *beautiful*
for putting a couple turns around a big fat probe. (They're supposedly
12uH/T^2, so 3T will get over 100uH easily!)

Tim
 
P

pawihte

Jan 1, 1970
0
Mark said:
accurately measuring the spikes at the output of a switchers is
an art
and a science.

The loop formed by the scope probe itself and the ground clip
can even
pick up spikes that are not really on the output.

Connect the scope probe to the ground where the ground clip is
connected and you should see nothing but you will probably
still see
the spikes. This indicates that you need to improve the
measurement.

I'm too lazy to search the web for you but you can look around
the web
yourself for techniques to measure ripple of switches and see
the
various techniques of filtering or common mode rejection that
are
suggested to get a more accurate measurement.

To be clear, I am saying that the spikes you are seeing on the
scope
are worse then are really there due to measurment technique
issues.

You're right. I touched the probe tip to the ground-clip and the
spikes remained almost the same. In fact, I should have thought
of this myself. Although it must be obvious that I have large
gaps in my knowledge, I've certainly been aware of such induced
pickups for a long time - decades actually. My only excuse is
that it was already the small hours of the morning when I ran the
trial. Thanks for pointing it out.

As a quick test, I placed the whole thing inside a tincan. I
wrapped the whole thing, including the mains transformer, in
bubble plastic wrapping without even grounding the circuit to the
can. Only the 9V output lines (7" of flex) and the mains wire
were outside. The spikes dropped from about +/- 2V to +/- 20mV !

I'll check out the suggestions made by others and report back.
 
P

pawihte

Jan 1, 1970
0
pawihte said:
You're right. I touched the probe tip to the ground-clip and
the
spikes remained almost the same. In fact, I should have thought
of this myself. Although it must be obvious that I have large
gaps in my knowledge, I've certainly been aware of such induced
pickups for a long time - decades actually. My only excuse is
that it was already the small hours of the morning when I ran
the
trial. Thanks for pointing it out.

As a quick test, I placed the whole thing inside a tincan. I
wrapped the whole thing, including the mains transformer, in
bubble plastic wrapping without even grounding the circuit to
the
can. Only the 9V output lines (7" of flex) and the mains wire
were outside. The spikes dropped from about +/- 2V to +/- 20mV
!

I'll check out the suggestions made by others and report back.

Correction: I was too quick to draw a conclusion. I took out the
whole thing from the tincan again, but the spikes are still only
+/-20mV. (This time I left the scope probe and ground clip
attached to the ends of the output wires). I further observed
that even slightly moving the setup (millimeters) caused the
spikes to shoot up again, and they remain high until I turn it
off and then on again. Even just touching the wires have an
effect. It seems something's unstable.
 
P

pawihte

Jan 1, 1970
0
pawihte said:
Correction: I was too quick to draw a conclusion. I took out
the
whole thing from the tincan again, but the spikes are still
only
+/-20mV. (This time I left the scope probe and ground clip
attached to the ends of the output wires). I further observed
that even slightly moving the setup (millimeters) caused the
spikes to shoot up again, and they remain high until I turn it
off and then on again. Even just touching the wires have an
effect. It seems something's unstable.

I increased the load to ~0.1A, then to 0.6A. The spikes remained
fairly stable at approx +/-10-20mV until I momentarily touch the
output +, at which point the spikes shoot up again until I turn
it off and on. Normal ripple, which was effectively nil at light
load, was about 5mVp-p at 0.6A.

Frequency is now about 45kHz. The ~15kHz I reported earlier was
probably an observation error on my part. There was a lot of
jitter. I think it's time to work on the inductors.
 
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